Modern materials present a number of formidable challenges to the fabricators of a wide range of optical, semiconductor, and electronic components, many of which require precision shaping, smoothing, and polishing. The use of plasmas to etch materials has become an important technique in the optical component and semiconductor industries. Recent advances have introduced sub-aperture plasma processes, such as reactive atom processing (RAP), which act more like traditional machining tools by etching only specific areas of a workpiece.
A plasma etching process differs from its mechanical counterpart by the mechanism in which material is removed. Traditional machine tools use mechanical parts to physically cut away material from a workpiece. Plasma etching processes, on the other hand, rely upon chemical reactions to transform the solid material of the workpiece into a volatile or otherwise labile byproduct. Plasmas offer advantages such as the contact-free removal of material, in which little to no force is extered on the workpiece. Reliance upon a chemical means of material removal introduces a whole new set of factors to consider when treating a material.
The activation energy (Ea) is an important factor to consider in any chemical reaction, as the activation energy is a type of ‘energy barrier’ for a reaction. Without sufficient energy, a given set of reactants will not react. Ea varies from reaction to reaction, and can be an important factor in determining the rate of a given chemical reaction at a specific temperature. The relationship between temperature, rate, and activation energy is described by the Arrhenius equation:k=Ze−Ea/RT where k is the reaction rate constant, Z is a proportionality constant that varies from reaction to reaction, Ea is the activation energy, R is the ideal gas constant, and T is the temperature.
For example, a process by which SiC is etched using F radicals will not produce any measurable material removal below a specified temperature, designated herein as temperature A. Given this situation, the traditional approach has been to increase the temperature of the entire SiC workpiece to a temperature of A or higher. This heating can be accomplished with the plasma torch itself, usually by a programmed preheating program, or by electric heating coils embedded in a temperature-controlled part chuck. Once the desired temperature is reached, and maintained, etching with the sub-aperture plasma can proceed similarly to scenarios where the material being etched has a negligible Ea. Some aspects of this process are problematic. When the workpiece is very large, supplying enough heat to evenly heat an entire workpiece can be problematic, especially when temperature A is large. Another drawback is the amount of time necessary to heat a workpiece, as heating a workpiece too quickly can induce thermal stresses which are undesirable in high precision components. In cases where the material requires a very high temperature, the part holder and chamber must be constructed of special heat-resistant materials. Additional systems must be added to the device in order to monitor and regulate such high temperatures.